1. Field of the Invention
The present invention relates to a sheet conveying apparatus for conveying a sheet such as an original, a recording sheet and the like, and more particularly, it relates to an original reading apparatus used with a facsimile system and the like.
2. Related Background Art
An example of a conventional original reading apparatus will be explained with reference to FIGS. 6 and 7. In FIG. 6, the reference numeral 51 denotes an original convey roller for conveying an original; 52 denotes an image sensor of complete contact type (referred to as "contact sensor" or "CS" hereinafter) acting as a reading device for reading image information on the original; 53 denotes a holder for locking and holding the CS 52; and 54 denotes an urging means for urging the CS 52 against the original convey roller through the holder 53. The holder 53 has a rotary support portion 53a for attaching the holder 53 to a body of the apparatus for a rocking movement, and a guide portion 53b for guiding the holder when the holder is engaged by the body of the apparatus.
When the original convey roller 51 is rotated in a direction (original conveying direction) shown by the arrow b, as shown in FIG. 7, in the case where the original 6 is pinched between the original convey roller 51 and the CS 52, the original 6 is subjected to a conveying force F1 (directing toward an original conveying direction shown by the arrow a in FIG. 7) from the original convey roller 51 due to friction between the original convey roller 51 and the original 6 and is also subjected to a resistance force F2 (directing toward a direction opposite to the original conveying direction a) due to friction between the original 6 and the CS 52. When F1&gt;F2, the original 6 is conveyed in the original conveying direction a. To this end, the coefficient .mu.1 of friction between the original convey roller 51 and the original 6 must be greater than the coefficient .mu.2 of friction between the original 6 and the CS 52 (.mu.1&gt;.mu.2). In order to surely convey various kinds of originals, a surface of the original convey roller 51 is constituted by elastic material such as rubber.
In the original reading apparatus shown in FIG. 6, when the original 6 is inserted into the apparatus, the original convey roller 51 is rotated in the original conveying direction b by detection of sensor(s) (not shown) and/or by manual input through an operation portion (not shown). When the original 6 is conveyed by the original convey roller 51, due to the friction between the original 6 and the CS 52, the CS 52 and the holder 53 for holding the CS are subjected to a conveying force F12 (in the original conveying direction a) corresponding to the above-mentioned resistance force F2, as shown in FIG. 7. By the way, the positioning of the CS 52 and the holder 53 for holding the CS regarding the original conveying direction a is effected by fitting a U-shaped notch 53c (FIG. 6) of the guide portion 53b of the holder 53 on a shaft 51a of the original convey roller 51.
However, in the above-mentioned conventional example, if there is no original 6 between the original convey roller 51 and the CS 52, as shown in FIG. 6, the CS 52 will be subjected to a conveying force F3 from the original convey roller 51 rotated in the direction b. Since an image information reading surface of the CS 52 is constituted by glass and the surface of the original convey roller 51 is constituted by rubber, the coefficient .mu.3 of friction between the original convey roller 51 and the CS 52 becomes greater than the friction coefficient .mu.2 (.mu.3&gt;.mu.2), thereby establishing a relation F3&gt;F2. Accordingly, if there is no original 6 between the original convey roller 51 and the CS 52, the conveying force acting on the CS 52 will be maximized.
Accordingly, if there is no original 6 between the original convey roller 51 and the CS 52, any load acting on an original convey driving system (not shown) such as a motor (drive source) for rotating the original convey roller 51 is also maximized. Since the U-shaped notch 53c of the guide portion 53b of the holder 53 is fitted on the shaft 51a of the original convey roller 51, the holder 53 is subjected to a force F4 (directing toward a direction opposite to the original conveying direction a) same as the above-mentioned conveying force F3 from the rotating shaft 51a of the original convey roller 51. However, in FIG. 6, since the guide portion 53b of the holder 53 is positioned above a point where the conveying force F3 of the original convey roller 51 acts on the CS 52 and the associated holder 53 (i.e., a contact point between the original convey roller 51 and the CS 52), couple of forces for rotating the CS 52 and the associated holder 53 around the rotary support portion 53a in a direction shown by the arrow c in FIG. 6 is generated by the above-mentioned forces F3 and F4.
Due to the generation of the couple of forces, the urging force of the CS 52 against the original convey roller 51 is increased. If such urging force is increased, the conveying force F3 received by the CS 52 from the original convey roller 51 is further increased, with the result that the couple of forces is further increased to further increase the urging force of the CS 52 against the original convey roller 51. As a result, the load acting on the drive system (not shown) of the original reading apparatus is greatly increased.
That is to say, in the arrangement of the conventional apparatus, when there is no original 6 between the original convey roller 51 and the CS 52 and the load acting on the drive system of the original reading apparatus is maximized, since the couple of forces is inevitably generated, thereby further increasing the load. As a result, the motor in the drive system is damaged or is out-of-phase. Further, with the arrangement as mentioned above, in FIG. 6, when an original discharge opening is provided at the left side of the original convey roller 51, it is necessary to reserve a space (for receiving the guide portion 53b) between the original discharge opening and the original convey roller 51, with the result that the entire apparatus is made bulky.